今日基础算法总结：归并排序

逆序对个数求解：

运用归并排序

#include<iostream>

using namespace std;

long long count=0;

const int N=1e6+10;

int n;
int q[N],temp[N];

void merge_sort(int q[],int l,int r)
{
if(l>=r) return;

int mid=l+r>>1;

merge_sort(q,l,mid);merge_sort(q,mid+1,r);

int k=0,i=l,j=mid+1;
while(i<=mid&&j<=r)
{
if(q[i]<=q[j]) temp[k++]=q[i++];
else
{
count+=mid-i+1;
temp[k++]=q[j++];
}
}
while(i<=mid) temp[k++]=q[i++];
while(j<=r) temp[k++]=q[j++];

for(int i=l,j=0;i<=r;i++,j++) q[i]=temp[j];
}
int main()
{
scanf("%d",&n);

for(int i=0;i<n;i++) scanf("%d",&q[i]);

merge_sort(q,0,n-1);

cout<<count;
}